Preferred embodiments of the present invention relate generally to a system and method for warning when a roundsling is loaded beyond its rated capacity, load limit or is potentially nearing failure. Preferred embodiments of the invention also relate to a system and method for monitoring loads applied to a roundsling in real time.
Industrial slings are typically constructed of metals or synthetic materials. Wire rope slings are commonly made of a plurality of metal strands twisted together and secured by large metal sleeves or collars. Synthetic slings are usually comprised of a lifting core made of strands of synthetic fiber and an outer cover that protects the core. The strands of the core are typically inserted in a generally parallel orientation to the other strands within the core, but may also be twisted as they are inserted into the cover, as is described in Slingmax's U.S. Pat. No. 7,926,859, which is incorporated herein by reference in its entirety. Synthetic slings provide weight, strength and other advantages over wire rope slings. One popular design of synthetic slings is a roundsling in which the lifting core forms a continuous loop and the sling has a circular or oval-shaped appearance.
Modern industrial slings may experience failure and loss of a load caused by the sling breaking or failing, for example, because the sling is fatigued, over-stretched or overloaded during a current or previous use. When subjected to an overload condition in excess of its rated capacity, a roundsling may be permanently damaged and/or deformed if the load stretches the fibers of the load bearing core material beyond their rated strength. When a synthetic fiber sling is overloaded beyond its tensile strength or weight-lifting capacity, it is considered to be damaged and may never return to its normal strength and load bearing capacity. Detection of such overloading conditions can be difficult to visually or otherwise inspect or determine during field use.
Slings are generally provided with specified load capacity (rated capacity), which is a load over which the particular sling should not be loaded. The rated capacity also provides guidance to users regarding the rated or safe lifting capacity of the sling. Nevertheless, this capacity is sometimes exceeded, either accidentally, by unexpected shock loading, or by users engaging in unsafe shortcuts during rigging and use of the sling. In addition, as the sling is used, it may become subject to abrasion, cuts or other environmental degradation to its fibers, which also weaken the working load limit, actual capacity and tensile strength of the sling and potentially negatively impact the rated capacity. Environmental factors that may weaken the working load limit, capacity and tensile strength of the sling include poor maintenance, ultraviolet radiation exposure, bending, kinks, knots, wear, fatigue, retention of water, temperature, and other related environmental factors. Individually or cumulatively, such conditions may lead to unexpected failure of the sling during use. It is, therefore, desirable to measure and record the loads that are applied to a sling every time the sling is used for lifting.
There are no methods known in the art for continuous, direct measurement of loads on either a wire rope or synthetic sling during industrial or field-use settings. Current methods rely on detecting only an overload condition or indirect measurements of loads, e.g., using load cells at attachment points or related measurement techniques. Depending on the rigging configuration, these indirect measurements may provide misleading information on direct loads applied to each independent sling that is used in a lifting job.
Often, over-load, fatigue, or damage to the sling materials are not readily apparent as the result of visual inspection, particularly given the large size or length of a particular sling, or because the load-bearing core is hidden inside the outer cover. If a roundsling is fatigued or structurally changed, the sling may no longer be able to lift a load according to its maximum rated load capacity or its load limit. These fatigue or structurally weakened conditions may become a threat to operators and riggers using the damaged sling.
A commercially available roundsling may include a pre-failure indicator. An example of such a pre-failure warning indicator is described in U.S. Pat. No. 7,661,737, the contents of which are incorporated herein by reference in their entirety. Such pre-failure indicators are designed to produce a visible sign of overload when the sling is overloaded beyond its rated capacity, but below its breaking strength. These pre-failure warning indicators do not determine the exact load imparted on the sling during loading, but only provide an indication that the sling was loaded beyond its rated capacity. In addition, depending on the rigging configuration and location of the sling or pre-failure indicator on the sling, it may be difficult for operators or riggers to visually identify the activation of the pre-failure indicators during the lifting operation. The inability to immediately identify the overloading condition might result in unsafe lifting operations continuing until the riggers inspect the roundsling after the lifting operation is completed.
There is a need in the art of rigging and sling inspection for consistent and reliable sling pre-failure indication. In addition, there is a need to identify structurally sound slings that have useful operational life even after their initially predicted lifetime. There is also a need to provide for structural health monitoring of the sling by monitoring the loads applied to the sling and the environmental exposure of the sling during operation to determine the state of the system health during the useful life or to more accurately predict the useful life of the sling. Finally, there is a need to measure loads that are imparted on slings in real time during lifting operations and to record and store loading information for individual slings over their lifetime to provide accurate and predictable useful life predictions for the slings. An advanced warning that a sling is near its breaking point provides operators of the sling with an opportunity to take corrective action. In addition, advanced warning of the structural capacity of the sling by monitoring and/or predicting the structural health of the sling can extend the lifetime of the sling, thereby reducing the necessity for costly and unnecessary replacement of the sling. Further, knowing the lifetime loading, environmental factors and overload history of a particular sling allows riggers to identify and select the safest and most appropriate equipment for each rigging task.